microsoft/compiler: Emit const accesses as load_deref

microsoft/compiler: Emit const accesses as load_deref

There's a few changes in here that are very inter-related.

First, we stop lowering load_deref on shader_temp to load_ptr_dxil,
and just leave it as load_deref. In order for that to work, we need
the derefs to be in a shape that's acceptable to DXIL, so the only
current producer of shader_temp loads (the CLC frontend) needs to
run some lowering passes on them first.

The DXIL backend is augmented to just write out deref indices while
walking a deref chain, which will get combined in the load op into
a GEP instruction. For non-mesh/raytracing shaders, these are required
to be single-level scalar arrays, but the complexity here is preparation
for when we don't need to do that anymore.

Additionally, the const lookups are changed from using a hash table
to just putting an index on the variable.

All of this together is enough to enable the authored-forever-ago test
which uses indirect array access into a const packed struct. The
load_ptr_dxil handling didn't deal with packed structs / unaligned
accesses, but now that we're in a logical address space with derefs
instead of physical, there's no alignment to deal with anymore and
the fact that it's packed goes out the window.

This removes one custom DXIL intrinsic.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23173>